Optimization of turbine blade tip clearances can lead to better engine performance and efficiency. To adjust the clearance between the tips of the rotating turbine blades and a shroud, an Active Clearance Control (ACC) system may provide thermal control air which impinges on the turbine casing with the intent of adjusting the position of the casing and shrouds relative to the blade tips. More particularly, an engine controller (e.g., an Electronic Engine Controller (EEC) or Electronic Control Unit (ECU)) equipped with Full Authority Digital Engine Control (FADEC) may utilize a clearance algorithm to calculate instantaneous turbine blade tip clearances. The calculated clearances may then be compared to a blade tip clearance target. If the calculated clearances do not align with the clearance target, the ACC system may adjust the blade tip clearances to force the calculated clearances to agree with the clearance target. In this way, the shrouds are adjusted relative to the blade tips.
Despite the ability of ACC systems to control blade tip clearances, clearance targets are typically set without regard to how an engine is actually or uniquely operated. Rather, each engine of a particular engine model targets the same blade tip clearances regardless of how the engine is operated. Blade tip clearance targets have conventionally been set such that an aircraft can accelerate from a given speed to a maximum continuous speed at any moment (i.e., a snap accel). In many cases, an aircraft is very unlikely to perform such a maneuver, and thus, the clearance between the shrouds and the turbine blade tips may be set unnecessarily open, leading to less than optimal engine performance and efficiency.
Therefore, improved active clearance control logic for adjusting blade tip clearances based on the way an engine is actually or uniquely operated would be desirable.